1. Field of the Invention
This invention is related to the field of fusion nuclear reactors and neutron generators. More specifically the invention is related to the "plasma focus" thermonuclear reactor in which a high speed pulsed electrical discharge heats, compresses, and concentrates a plasma of thermonuclear fuel at a focus point within a reaction chamber.
Fusion reactors may burn deuterium, a naturally occurring isotope of hydrogen which is easily obtained from water, and do not produce radioactive waste. Fusion reactors cannot melt down. The high temperature of the fusion process permits increased conversion efficiency to electrical power, including the possibility of direct conversion from microwaves generated by the plasma ions. Neutrons are a useful byproduct of the deuterium fusion reaction and may be used to convert lithium to tritium, fertile uranium to plutonium, thorium to fissile uranium, or radioactive waste to less harmful isotopes. But unlike fission reactors, fusion reactors need not produce neutrons if a boron-hydrogen fuel is used.
Plasma focus fusion reactors offer several inherent advantages over magnetic confinement schemes using external superconducting magnets or electromagnets. The pressure of the gas fuel in the reaction chamber may be on the order of atmospheric pressure and thus high-vacuum seals are not needed. The power density of the reactor is much higher and thus the reaction chamber may be much smaller. Since the electrical discharge provides the confining magnetic field and simultaneously heats the plasma, there is no inherent need for external means for heating the plasma to the reaction temperature, and there are no external electromagnets with resistive losses. The confining magnetic field of the electrical discharge is also much higher than the maximum possible fields produced by superconducting magnets, and thus there is no inherent need to burn a tritium-deuterium fuel which fuses at the lowest temperature and density-confinement product. The use of tritium is undesirable since tritium is an artificial radioactive isotope.
2. Description of the Prior Art (Rule 97 Prior Art Statement)
A summary of the prior art for magnetic confinement fusion reactors in general is provided in E. Lerner, "Magnetic Fusion Power," IEEE Spectrum, Vol. 17, No. 12, December 1980 at 44. Mr. Lerner discloses that controlled thermonuclear fusion power using magnetic confinement now seems practical, but the best method remains uncertain. Mr. Lerner further discloses that the plasma focus is undoubtedly the simplest of all fusion devices, and neutron energy released by the focus increases as the fifth power of the input current, implying that breakeven devices could be built that use relatively small energy sources of 4 MJ. The Soviet Union has taken a keen interest in the focus and is currently building a 2.5 MJ device at the Kurchatov Institute, which is at least ten times larger than focus devices under construction in the United States.
Mr. Lerner's article also discloses a reasonable explanation of why the focus device, consisting of two oppositely charged coaxial electrodes, achieves appreciably better results than pinched sparks in general. In the focus device current filament pairs roll down the length of the coaxial electrodes and, at the end of the electrode, begin to merge and annihilate each other. The extremely rapid change of magnetic field induces very high electric fields, over 100 MV/cm, accelerating ions to 300 to 400 keV. The resulting high current, carried by the few survivor filaments, creates compressional fields of 60 KT or more, compressing the plasma to densities of 10**20/cm**3. For several nanoseconds, thermonuclear reactions occur within the filaments, about 20 micrometers in diameter, which in turn are composed of even smaller components only a micrometer in diameter.
Plasma focus fusion devices at first used hemispherical electrodes which were electrically connected via a coplanar, circular plate transmission line to a bank of capacitors generally radially disposed about the electrodes. An excellent summary of plasma focus devices and design techniques circa 1965 is provided in F. Frungel, High Speed Pulse Technology, 1965 Academic Press, Lib. Cong. 65-16665, Vol. 1 & 2 at 36-44, 393-401, FIGS. A7-3, A8-1, G8c-2. Frungel teaches that the leads that carry the current from the capacitor to the discharge gap should be designed to keep the parasitic inductance of the circuit down to a minimum since the parasitic inductance restricts the magnitude of the current and its rate of growth. The problem of obtaining low inductance in parallel-connected capacitors lies not in the capacitors themselves but in the arrangement of the connections. Bifilar construction of the bus bars is necessary. Current growth up to 10**12 amp./sec. is possible.
Normally the generation of "pinched" sparks, in which a plasma is heated and compressed by a fast increase in current density, requires at least 5 KJ of stored energy, but H. Fischer designed an electrical discharge device for photographic work with an extremely small circuit inductance that achieved a pinched spark with only 0.05 J. A generally cylindrical, coaxial capacitor formed in the shape of a test tube was provided with an aperture at the bottom at which a electrical discharge occurred at an electrode protruding through the aperture. H. Fischer, "Millimicrosecond Light Source With Increased Brightness," Journal of the Optical Society of America, Vol. 51, No. 5, May 1961, at 543.
After 1965 several very large megajoule capacitor banks were constructed for experimental use with induced field pinch devices, in which the capacitor bank is discharged into an external electromagnet. Switching is permormed by a spark gap switch on each capacitor, connecting the capacitor to a coaxial transmission line. The transmission lines are terminated at right angles to and above and below a circular plate transmission line feeding a centrally located fushion device. Survey of the USAEC Program in Controlled Thermonuclear Research, 2ed, U.S. Atomic Energy Com'n, Division of Controlled Thermonuclear Research, at 26-31 (WASH 1277 UC-20).
T. Roberts et al., U.S. Pat. No. 3,766,004 issued Oct. 16, 1973, discloses the use of a carbon dioxide laser to provide additional heating of the plasma in a plasma focus device and also discloses a focus device with a circular plate geometry, in FIG. 2. Roberts finds the circular plate geometry useful for localizing the focus point so that it is collinear with the axis of the laser. Roberts also discloses that even relatively small increases in the temperature of the plasma yields a significant increase in the rate of neutron production.
Electrical discharges have also been useful for machining. E. Williams and C. Porterfield, U.S. Pat. No. 2,979,639 issued Apr. 11, 1961 discloses increasing the capacitor voltage as an attractive approach to higher power and metal removal rates.